1. Field of the Invention
The invention relates to a method for pulling a single crystal composed of silicon from a melt contained in a crucible, wherein the single crystal, during pulling, is surrounded by a heat shield, the lower end of which is at a distance h from the surface of the melt, wherein a gas flows downward in the region between the single crystal and the heat shield, outward between the lower end of the heat shield and the silicon melt and then upward again in the region outside the heat shield. The invention also relates to a single crystal which can be produced by the method.
2. Background Art
It is known that the Czochalski method of growing single crystals is a very sensitive process and crystal structural defects in the form of dislocations and atomic point defects can occur. In addition, the occurrence of macroscopic voids is observed, the latter being bubble-type holes having diameters of a few micrometers up to millimeters. These should not be confused with COPs (“crystal originated particles”), which are formed from agglomerated atomic vacancies and which have a diameter of up to a few hundred nanometers.
The incorporation of voids into the growing single crystal does not inevitably lead to the formation of dislocations and therefore generally remains undetected during the crystal pulling process. It is only when the single crystal is subsequently sliced into wafers that the voids included therein can be detected by a visual inspection. In general, only relatively few silicon wafers (in the per mille range) are affected which are rejected by the visual inspection. However, frequent occurrence is observed in the case of specific important production processes, e.g. in the case of highly doped crystals. Moreover, it is difficult to detect small voids with hundred percent certainty. There is the risk of voids within the silicon wafer remaining undiscovered during the visual inspection. For component-manufacturing customers, however, it is of crucial importance to acquire only wafers which have no voids of this type. The uncontrolled occurrence of macroscopic voids can lead to huge economic losses. It is therefore desirable and necessary to find solutions which significantly reduce or even completely prevent the harmful voids.
In EP756024A2 it is emphasized that it is advantageous initially to melt fragments composed of silicon, rather than silicon granules, because the latter, on account of a high hydrogen content, tend to form bubbles which can ultimately be incorporated into the single crystal.
U.S. Pat. No. 5,902,394 describes a method which is intended to drive the gas bubbles out of the melt prior to crystal growth and substantially consists of varying the rotational speed of the crucible.
In U.S. Pat. No. 6,086,671 gas bubbles in the melt are mentioned as a cause of dislocations. They can be suppressed by a static magnetic field being applied as early as during the melting of the polycrystalline material.
DE 102007023040A1 describes a pulling method in which the melt is degassed prior to the attachment of the seed crystal by increasing the temperature and applying a static magnetic field. The duration of this degassing phase is up to two hours.
U.S. 2008/0011222A1 also assumes that the macroscopic voids are caused by gas bubbles which arise during the melting of the polycrystalline semiconductor material in the quartz glass crucible. An improved melting process is accordingly proposed, starting with melting by means of a side heater. The melt is then heated further in the bottom region of the crucible, which drives the melt movement and is thereby intended to lead to the degassing of the melt.
It has been found, however, that the occurrence of voids cannot be completely avoided by means of the described methods for carrying out the melting phase.
In accordance with U.S. Pat. No. 6,340,390 B1 the pressure in the chamber of the crystal pulling installation during crystal pulling is kept below 95 mbar and below the pressure prevailing during the melting phase. The low pressure during crystal pulling is intended to lead to continuous degassing of the melt and thus to avoidance of the incorporation of gas bubbles into the growing single crystal. However, the pressure is an important parameter for the entire crystal quality. By way of example, the oxygen content of the single crystal and, consequently, the precipitation capability and thus the gettering capability are altered. Reducing the macroscopic voids by correspondingly setting the pressure is therefore associated with disadvantages. Moreover, the effect described is not always obtained in practice.